The present invention relates to a transmission power amplification method and apparatus which realize transmission power control suitable for a case where a transmission power amplification section includes a nonlinear gain amplifier.
In a recent digital mobile communication system, a next-generation portable telephone scheme based on a W-CDMA (Wideband-Code Division Multiple Access) scheme using a spread spectrum scheme has proceeded toward commercialization in order to further improve the frequency usage efficiency. According to the W-CDMA scheme, to solve the so-called near-far problem, a dynamic range of 70-odd dB or more needs to be quickly changed with high precision in transmission power control.
Even if the use of a Class-A or Class-B power amplifier used in a transmission power amplification section is limited to an operating point at which the amplifier exhibits excellent linearity, transmission is not frequency performed at the maximum output in practice, and power is always consumed even in the low-transmission output mode owing to DC bias currents. Demands have therefore arisen for improvements in the power conversion efficiency of these nonlinear gain amplifiers.
As means for improving the power conversion efficiency of a nonlinear gain amplifier using an FET (Field Effect Transistor) element, a bias control method, a method of bypassing a nonlinear gain amplifier by using a switch, and the like are available. FIG. 5 shows the relationship between the transmission output of a nonlinear gain amplifier and the power conversion efficiency in a case where the drain voltage is so controlled as to optimize the power conversion efficiency in consideration of the operation stability of the nonlinear gain amplifier. As shown in FIG. 5, the power conversion efficiency greatly improves when the drain voltage is controlled as compared with a case where the drain voltage is not controlled.
If, however, the drain voltage is low, the linearity of the nonlinear gain amplifier deteriorates, and its operation becomes unstable. When a nonlinear gain amplifier is to be bypassed by using a switch, the power consumption of the nonlinear gain amplifier can be reduced to zero by turning off the power supply of the nonlinear gain amplifier. However, the transmission output level becomes discontinuous in switching operation.
Japanese Patent Laid-Open No. 10-294626 (reference 1) discloses a gain controller for reducing discontinuous portions of the transmission output level by bypassing the nonlinear gain amplifier using a switch. FIG. 6 shows a gain controller disclosed in reference 1. Referring to FIG. 6, a signal input from an input terminal 501 is branched into two paths by a distributor 510 and respectively input to first and second variable gain amplifiers 516. An output from the first variable gain amplifier 511 is amplified by a linear power amplifier 514 through a filter 512 and driver amplifier 513. The resultant signal is output to a synthesizer 515.
The synthesizer 515 synthesizes the output from the second variable gain amplifier 516 with the output from the linear power amplifier 514. The resultant signal is output from an output terminal 502. A voltage from a power supply 503 is applied to the linear power amplifier 514, driver amplifier 513, and first variable gain amplifier 511 through a switch circuit 518. A control circuit 517 controls the first variable gain amplifier 511, second variable gain amplifier 516, and switch circuit 518.
The operation of the gain controller having the above arrangement will be briefly described next. If a desired transmission output is high, the gains of the first and second variable gain amplifiers 516 are set to be large and small, respectively. In this case, a main signal is transmitted through a first path formed by the first variable gain amplifier 511. If a desired transmission output low, a main signal is transmitted through a second path formed by the second variable gain amplifier 516.
When the gain level of the first variable gain amplifier 511 becomes sufficiently lower than that of the second path, the switch circuit 518 is switched to turn off the power supply for the first variable gain amplifier 511, driver amplifier 513, and linear power amplifier 514, thereby suppressing their current consumption to 0.
That is, two parallel paths are independently gain-controlled such that a route exhibiting an optimal power efficiency is selected in accordance with a required transmission output level, and the gains of the respective paths are continuously switched/controlled to reduce discontinuity.
In the above conventional gain controller, however, when outputs from the first and second paths are to be synthesized, since the first and second paths are connected in parallel, the gain values of the respective paths which are required to obtain a desired transmission output level cannot be obtained by simply adding the gain of the first path to the gain of the second path. That is, the levels of the respective paths must be inversely calculated such that the level after synthesis becomes the desired transmission output level, and the necessary gains of the respective paths must be calculated from the inversely calculated levels.
In a digital mobile communication system in which transmission power must be variably controlled with high precision at high speed with respect to an ambient radio environment that changes incessantly in order to cover a wide dynamic range, a complicated algorithm is required for transmission power control, and much time is required for processing.
It is an object of the present invention to provide a transmission power amplification method and apparatus which can ensure an optimal power conversion efficiency and linearity in obtaining a desired transmission power.
In order to achieve the above object, according to the present invention, there is provided an transmission power amplification apparatus comprising first variable gain amplification means for amplifying an input signal, second variable gain amplification means for amplifying an output from the first variable gain amplification means, nonlinear gain amplification means for nonlinearly amplifying an output from the second variable gain amplification means and outputting a transmission signal, and control means for controlling amplification of the input signal by using the synthetic gain of the first and second variable gain amplification means and the nonlinear gain amplification means in a transmission power range from a maximum value to an intermediate value, and controlling amplification of the input signal by using only the gain of the first variable gain amplification means in a transmission power range from the intermediate value to a minimum value, wherein the second variable gain amplification means has a gain characteristic that corrects nonlinearity of the nonlinear gain amplification means.